Surfactant flooding has become one of the more promising enhanced oil recovery techniques for recovering oil after water flooding. Generally, the method of surfactant flooding employs the injection of a surfactant in an aqueous solution or in an emulsion to sweep through the formation and recover oil. It is believed that surfactants are effective because of their ability to reduce the interfacial tension between crude oil and water. This reduction of interfacial tension permits the creation of various oil, water and surfactant emulsions in the formation. Thus, the produced liquids of surfactant flooding are generally stable oil, water and surfactant emulsions.
Because of the relative amounts of oil and water produced in chemical flooding, the produced emulsions will usually be oil-in-water emulsions. Upon settling, such emulsions may change to the water-in-oil emulsions which are usually encountered in primary petroleum production. Conventional surfactant extraction and emulsion breaking techniques which work on water-in-oil emulsions may be ineffective with oil-in-water emulsions.
Second, the creation of these generally highly stable emulsions results in decreased surfactant reutilization efficiency. Considerable quantities of surfactants are entrained in the produced emulsions which prevents the effective reutilization of the produced surfactant in the continuing surfactant flooding process. Because surfactants are expensive additives for chemical floods, commercial economics require that surfactant costs be minimized by reutilizing a substantial quantity of surfactant that is injected into the formation. The produced emulsions cannot be recycled because the surfactants in the emulsions are too diluted in aqueous volume and frequently, the emulsions are not at the preferred salinity levels. Surfactants injected into a reservoir must not be contaminated with materials that lower the effectiveness of the surfactant.
Third and most importantly, the presence of surfactant in the produced emulsion fluids makes it very difficult to recover oil free of impurities such as surfactant and water. All of the below cited references have attempted to solve the problem of freeing up salable oil from emulsions. One reference, U.S. Pat. No. 4,277,352 has also attempted to recover surfactant for reutilization, but none of the references have addressed the problem of recovering a surfactant free of contaminants which decrease the effectiveness of a surfactant when it is reutilized.
One method of breaking an oil-in-water emulsion is described in U.S. Pat. No. 4,029,570 where an additional formation brine is added to the produced emulsion to produce an oil phase and a water phase. However, such brine addition treatments will in many cases not work on produced fluids. One reason is the tendency of formation rock to selectively adsorb high equivalent weight sulfonates, leaving lower equivalent weight sulfonates behind, rendering the surfactant highly hydrophilic. Such processes will not sufficiently extract and concentrate the surfactant to allow reutilization. The surfactant is often left highly diluted in a relatively large volume of oil-in-water emulsion unsuitable for reutilization.
U.S. Pat. No. 3,637,521 discloses an emulsion breaking process comprising the steps of (1) adding an acid to lower the pH of the emulsion to approximately 5.0 and (2) adding an alkylamine containing 1 to 5 nitrogen atoms and from 2 to 12 carbon atoms. U.S. Pat. No. 3,687,845 describes a process wherein oil-in-water emulsions are treated by the addition of a high molecular weight, watersoluble polymer such as the polyvinyl aromatics of benzene such as polystyrene, polyvinyl toluene and several acrylamide compounds.
Another method which has been tried to break oil-in-water emulsions is the use of additional surface active agents. U.S. Pat. No. 4,261,812 discloses a method which employs an additional surface active agent having an average equivalent weight higher than the equivalent weight of the surface active agents in the produced emulsion, preferably an equivalent weight of about 400 to 600. U.S. Pat. No. 4,073,344 mentions the use of dimethylbenzene ammonium chloride, trimethyl ammonium chloride and alkyl benzene sulfonates for the same purpose. The use of additional solubilizers to break oil-in-water emulsions was promoted by U.S. Pat. No. 4,277,352 which suggests adding materials commonly used as solubilizers in surfactant flooding to the produced emulsion.
Although several of the above discussed methods may provide a way to free crude oil from produced emulsions, none of the references provide a process in which the surfactant can be economically recovered and concentrated for reuse.